Hi All,
I have a Kindle paperwhite that I would like to change the font on. I have down loaded some unlocked books to Calbre and changed the format to Open Dyslexic and this worked great on my computer. When I try and down load the changed document to my Kindle, the format is changed back to the standard Kindle format. Can anyone provide some answers or support?

It might still make a difference on how you transfer the book to the Kindle. I had trouble earlier on when I tried side loading kf8 books to Kindles and other devices with the Kindle apps. Sometimes they were viewed as kf8 books, often they were not. I have much better luck transferring kf8 books via Send to Kindle which uses the Kindle Cloud.

Another oddity is how various Kindles and devices interpret embedded fonts. Each seems to do its own thing and the results vary between devices. For example, the embedded fonts are currently broken on the latest Kindle for iPad app, work very well on the Fire HD, and works so so on the Kindle Touch.

As alternative to embedding a font into a KF8 file, you can also install custom font(s) on the PW. No jailbreak is needed on 5.3.1 and newer. The fonts will be available on all books as part of the regular font-selection screen (aA). This includes old mobi and new mobi (KF8) files. All you need to do are these steps:

The newly installed font will be visible (among a few others previously unavailable) in the font menu of a book. The only exception where it won't work is some KF8 books that have the font annoyingly hardcoded into the book (font won't change no matter which font is selected). The USE_ALT_FONTS will work on non-jailbroke PW, but firmware 5.3.1 or 5.3.3 is necessary. (see Note2)

How many fonts?: You can put up to 10 custom fonts on PW and still be able (barely) to change the settings for line-spacing and margin-size. When using KF8 books alot, it is better to limit the custom fonts to 9, as an embedded font will take up one spot. To avoid partially cutting off the icons on bottom of aA-menu use 6 or less custom fonts.

How to hide extra fonts?: It is not possible to hide fonts that show up extra, unless you edit a config file that can only be accessed with jailbreak. WARNING: This is dangerous, use at your own risk, link to instructions here.

Kindle Touch: This works on (non-)jailbroken KT running new FW 5.3.2 and for older FW down to 5.1.0. On KT (menu->settings) above only works from home screen - on PW the settings can be entered even inside a book.

Note1: Full restart is not necessary, ;fc-cache in search bar should be enough. When I tested on my PW the restart was faster than waiting on ;fc-cache to be done fiddling around. Please see eureka's post mentioned in Credits for more details.

Note2: It may or may not work on 5.3.0 non-jailbroke, but does possibly work jailbroke. Also if it works on non-jailbroke FW, it will still work if jailbroke.

Note3: fonts/ mentioned is /mnt/us/fonts/ when you have access to the complete filesystem (needs jailbreak)

Note4: You don't have to put all 4 styles of a font into the fonts/ folder. Unexpected results may occur though if you don't. Most of the time the Kindle will display the font automatically slanted for italic if italic font-file is missing, or substitute bold with the regular version. Other times a missing style might be replaced with a fallback font (e.g. Caecilia). There might be a logic behind it, but only Amazon will know. Some fonts will not work and crash the Kindle (offending meta-data in font ). When that happens, after it restarted, simply remove the last added font and it will be back to normal after restart or maybe ;fc-cache.

Credits: I have merely written this guide in easy to follow steps. The real work of the discovery of USE_ALT_FONTS was done by eureka and first mentioned by him in this post for the Touch. If you find my guide useful, then please also stop by eureka's post and give him a nice chunk of Karma to say:

As alternative to embedding a font into a KF8 file, you could also install a custom font on the PW that will be available on all books as part of the regular font-selection screen. This includes old mobi and new mobi (KF8) files. All you need to do are these steps:

The newly inshalled font will be visible (among a few others previously unavailable) in the font menu of a book. The only exception where it won't work is some KF8 books that have the font annoyingly hardcoded into the book (font won't change no matter which font is selected). The USE_ALT_FONTS will work no matter if the PW is jailbroke or not, but firmware 5.3.0 or 5.3.1 is necessary.

Great!!

It works whith more than one fonts?, or iT only can work with lonely alternative font...

As alternative to embedding a font into a KF8 file, you could also install a custom font on the PW that will be available on all books as part of the regular font-selection screen. This includes old mobi and new mobi (KF8) files. All you need to do are these steps:

The newly inshalled font will be visible (among a few others previously unavailable) in the font menu of a book. The only exception where it won't work is some KF8 books that have the font annoyingly hardcoded into the book (font won't change no matter which font is selected). The USE_ALT_FONTS will work no matter if the PW is jailbroke or not, but firmware 5.3.0 or 5.3.1 is necessary.

It works fine!!.

But I copied two fonts in the folder, and when I touch Aa, show me 20 fonts

It works whith more than one fonts?, or iT only can work with lonely alternative font...

No it works with 8 alternate fonts feasible, leaving room for 2 more for a total of 10 custom fonts. I suggest you never add more than 8 though, as adding an extra one would make it hard (not impossible though) to push the bottom buttons. Another reason to leave room for extra is publisher fonts - for books with embedded font.

I made a screenshot with 8 custom fonts - this way you can see what I mean by cutting off the buttons. See this post - you can also see the fonts available additionally that I did not add myself.